1. Technical Field
This disclosure relates to energy dissipation devices which can be used in structural engineering.
2. Description of the Related Art
Often, it is necessary to control the physical response of a mechanical or structural system from some dynamic excitation. This can be done through the use of materials with inelastic behaviors. In buildings, for instance, certain regions of the building may be designed as “structural fuses” that become damaged during a high-intensity seismic event. These “fuses” can be constructed of steel or reinforced concrete and detailed such that they deform in a ductile manner, dissipating energy during a dynamic motion. An example of one of these “fuse” types is the braced frame. There are several well developed, and quite effective, types of structural “fuses” used in structural and mechanical system design. Most have a similar behavior when loaded in either of two possible directions (e.g. tension/compression, positive/negative bending, etc.) This type of behavior is described herein as having a symmetric load-deformation behavior.
Some applications have a need for an asymmetric load-deformation behavior. As an example, a pre-engineered metal building can use steel moment frames to resist lateral building loads in the building transverse direction. Most metal buildings are lightweight, and thus their structural member sizes are controlled by load demands from non-seismic types of loading, such as snow or roof dead load. Research has shown that these types of structures lack the level of ductility often found in traditional column-and-beam steel frames. An attempt at using a traditional steel plastic hinge would place a symmetric behavior hinge near the column-to-rafter joint, but as the bending moments from snow and dead load often exceed those from the expected seismic demands, sizing the plastic hinges for seismic demands would result in failure under those other loadings. Stronger plastic hinges, sized for the controlling load cases, would result in a lack of energy dissipation during seismic excitation. Therefore, this application would benefit from an energy dissipation device that would provide sufficient strength and stiffness to resist non-seismic loads, while still deforming significantly under seismic demands, such as an energy dissipation device with an asymmetric load-deformation behavior.
The concentric braced frame is an example of an energy dissipation system that has an asymmetric load-deformation behavior. It is designed to yield in tension. However, it buckles in compression. Due to the high local strains caused by buckling, cyclic loading of the concentric braced frame tends to result in low-cycle fatigue cracking. Also, the post-buckling strength and stiffness of the brace is difficult to predict and typically a source of high uncertainty in design.
Another type of lateral load resisting system formed from asymmetrically behaving components is the tension-only brace. This type of brace, made from slender rods or cables, has effectively no compression strength, and therefore must be used in pairs (e.g. an X-configuration) to provide bi-directional strength and stiffness. The tension-only braces do not typically provide energy dissipation, only linear elastic strength and stiffness to resist lateral forces on the buildings. If they were to yield, the rods/cables would elongate and effectively result in an unsupported structure when the building returned to its original configuration. Neither of these types of devices provides a reliable and ductile asymmetric load-deformation behavior.